Baseline Calibrations

For all queue
observations, a set of standard calibrations (the "baseline
calibrations") will be taken by Gemini Staff
to ensure the long-term utility of data in the archive. The baseline
calibration set varies from instrument to instrument and from mode to
mode. The table below describes the GSAOI baseline calibrations, as well
as provides information on how and when to request additional frames.

All GSAOI baseline calibrations are taken from the GS-CALYYYYMMDD (shared)
program, and not from the individual science programs. This means the
users need to search for the calibrations separatelly when downloading
their science data from the GSA. Instructions/tutorials on the
procedures can be found in the "GSA Search and Download Instructions" page.

GSAOI Baseline Calibrations

Measurement Type

Notes

Bad pixel mask

Derived each semester
from GCAL H band flats and dark images. The GSAOI detectors have less
than optimal cosmetic quality, due to the presence of "dead" pixels,
light-emitting pixels, "killed" spots and a significant number of hot
pixels. All of these are found to be quite stable, but the number of
hot pixels detected (for a given cut level above the mean value in the
dark frame), is dependent on the exposure time of the dark frames used.
The standard BPM included in the Gemini GSAOI IRAF package is defined
using 150sec darks. If desired, the applicants can require to take one set of 10-15 frames (darks + flats) per semester using darks with exposure times/filter tailored to their program in order to construct a BPM. Flats can be either GCAL or domeflats. A document describing the process followed to derive the standard BPM is now available.

Flat field

The baseline flat fields for GSAOI are taken as dome flats
for all broad band and narrow band filters. The current calibration
plan is to obtain two sets of 10 flats per filter per run (at the start
and at the end of the laser block). Pending analysis of the flat
stability, this can be reduced to one set per run (the data so far
indicate that flats are quite stable on periods of months). OFF flats are taken for the H and K band filters.

GCAL flats can be obtained by request in all filters but H, K, Ks, Kp
and CO 2.36mu - GCAL flats in these five filters are rendered useless
due to structure originated in the GCAL ND filter. Users are reminded
that GCAL flats do not correspond to the same optical path as the
science frames (the GCAL light does not pass through the GeMS AO bench).

Twilight flats in the broad band filters only (Z, J, H, K, Ks, Kp) will
be taken as often as possible (we aim at a complete set per laser run).
These will be evening twilights. We do not plan to take twilights in the
narrow band filters as part of routine baseline calibrations, as these
do not show any significant difference against the domeflats.

All flats are taken using the Bright Object read mode.

Fringe Frames

Not applicable

Dark / Bias

The dark current for
GSAOI is of the order of 0.01 e-/s/pixel, implying on a total dark
charge of ~12e- for the longest integration time allowed (20min). This
very low dark current implies that daily darks taken with the same
exposure time as the science frames (for dark subtraction as part of the
standard reduction process) are NOT necessary, and in fact, subtracting
a dark frame from the science and sky frames prior to subtracting the
sky from the science proper will result in increasing noise. Both the
dark current and the cross-shaped "warm" pixels are accounted for when subtracting the sky.

Darks (or rather "flats-off") are not really required for Dome or Twilight
Flats - this was tested during commissioning and attempting to subtract
darks or flats-off from twilight and dome frames, respectively, have not
produced any measurable improvement in the quality of the resulting
flat-field. By request of users, however, domeflats OFF are taken for the filters in the H and K bands. GCAL flats-off can be obtained as part of the GCAL flat
sequence, if such is required (but users are reminded that GCAL flats
are sub-optimal for data reduction).

If the applicant deems that darks are required for the data reduction, a
daytime sequence can be added to the PhaseII and will be taken on a
best effort basis within a few days of the observation execution (not
charged to the programme).

Given the low dark current and the temporal behaviour of the dark
frames, current plans are to create a library of dark frames with
different exposure times, which can be downloaded from the GSA and used
for data reduction if necessary.

Wavelength calibration

Not applicable

Telluric standard star

Not applicable

Flux standard stars

Selected from the Persson (1988) list. Observations of standard stars are only obtained in
broadband filters J, H, K, Ks, Kp. A more limited set of standards include the Z filter. Our goal is to obtain at
least 2-3 standards at different airmasses each observing night.
Approximate zero points are also available, and an study of
the color effects originated from the differences in QE between the four
arrays is ongoing. Photometric accuracy is limited to 5-10% due to uncertainties in the atmospheric extinction over Cerro Pachon, flat-fielding and sky subtraction, and the precision in the gain determination for each of the arrays. If the user is aiming at a higher photometric precision, it is necessary to self-calibrate the images, detector by detector.

Users should be aware that the standard star observations are NOT performed with the GeMS loops closed (NO AO correction - natural seeing only).
If the applicants would like to obtain the flux calibration stars with
the AO loops closed, these should be provided as separate targets in
PhaseI and included as Program Standards in PhaseII, therefore incurring
in charged time to the program. The same procedure should be followed
if the program requires a specific calibrator to be observed (e.g., for
narrow band filters). Users should be careful to select standards which will NOT saturate in the fully corrected images, and check if the guide stars can be mantained for all offset positions. Please remember that the ODGW guide stars cannot be moved from one detector to another within the same sequence.

Atmospheric extinction

Not included as part of
the baseline calibration set.

Point Spread Function star

Not included
as part of
the baseline calibration set. Applicants are reminded that the PSF
in an MCAO-corrected image is dependent on the position in the field AND
on the geometry of the guide star constelation (plus higher order terms
such as laser power and return, relative brightness of the guide stars,
etc). There is probably little scientific gain in attempting to derive a
PSF from a separate star field.

World coordinate system

Automatically
included for each GSAOI image. Relative accuracy is currently ~0.3arcsec
on average over the field. Absolute accuracy is around 5arcsec (limited by the precision in the telescope pointing).